Direct Measurement of the Return Current Instability in a Laser-Produced Plasma.

Area of Science:

• Plasma Physics
• Instability Studies
• Nonlocal Transport Phenomena

Background:

• The return current instability is crucial for understanding energy transport in plasmas.
• Classical theories like Spitzer-Härm may not fully capture plasma behavior under certain conditions.
• Nonlocal transport effects become significant when particle orbits are not localized.

Purpose of the Study:

• To measure the growth rate of the return current instability.
• To investigate the concurrence of this instability with nonlocal transport.
• To validate theoretical models with experimental data.

Main Methods:

• Experimental measurements of instability growth rate using Thomson scattering.
• Analysis of plasma conditions to determine heat flux characteristics.
• Vlasov-Fokker-Planck simulations incorporating nonlocal effects.

Main Results:

• A maximum growth rate of 5.1×10^9 Hz was measured for the return current instability.
• The measured growth rate was approximately three times lower than predicted by classical Spitzer-Härm theory.
• Experimental data confirmed that heat flux was nonlocal under the measured plasma conditions.
• Vlasov-Fokker-Planck simulations successfully reproduced the measured growth rates.

Conclusions:

• The return current instability is directly linked to nonlocal transport in plasmas.
• Experimental results align well with theoretical predictions, particularly regarding the instability threshold.
• The study highlights the importance of nonlocal effects in accurately modeling plasma behavior.